<!DOCTYPE html>
<html lang="zh-CN">
  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width,initial-scale=1">
    <title>第十章 函数 | 伊水河的博客</title>
    <meta name="generator" content="VuePress 1.7.1">
    <link rel="icon" href="/logo.png">
    <meta name="description" content="SO FAR SO GOOD">
    <meta name="twitter:title" content="第十章 函数">
    <meta name="twitter:description" content="">
    <meta name="twitter:card" content="summary">
    <meta name="twitter:url" content="https://yihe.cool/note/professionalJavascript4/10.html">
    <meta property="og:type" content="article">
    <meta property="og:title" content="第十章 函数">
    <meta property="og:description" content="">
    <meta property="og:url" content="https://yihe.cool/note/professionalJavascript4/10.html">
    <meta property="article:author" content="伊水河">
    <meta property="article:published_time" content="2020-12-18T00:00:00.000Z">
    <meta property="article:tag" content="javascript高级程序设计">
    <meta itemprop="name" content="第十章 函数">
    <meta itemprop="description" content="">
    <meta name="viewport" content="width=device-width,initial-scale=1,user-scalable=no">
    
    <link rel="preload" href="/assets/css/0.styles.152c4062.css" as="style"><link rel="preload" href="/assets/js/app.2d22f3f9.js" as="script"><link rel="preload" href="/assets/js/3.6721f6a0.js" as="script"><link rel="preload" href="/assets/js/1.546ea5a7.js" as="script"><link rel="preload" href="/assets/js/44.3033d12c.js" as="script"><link rel="prefetch" href="/assets/js/10.b827ae65.js"><link rel="prefetch" href="/assets/js/11.5e38b612.js"><link rel="prefetch" href="/assets/js/12.3f3699b0.js"><link rel="prefetch" href="/assets/js/13.7d83bc51.js"><link rel="prefetch" href="/assets/js/14.2ea90c77.js"><link rel="prefetch" href="/assets/js/15.7df6d74c.js"><link rel="prefetch" href="/assets/js/16.33c1bac8.js"><link rel="prefetch" href="/assets/js/17.1aef86d1.js"><link rel="prefetch" href="/assets/js/18.b74cd2bd.js"><link rel="prefetch" href="/assets/js/19.73f82cea.js"><link rel="prefetch" href="/assets/js/20.6354096e.js"><link rel="prefetch" href="/assets/js/21.fd87273b.js"><link rel="prefetch" href="/assets/js/22.f90ee83e.js"><link rel="prefetch" href="/assets/js/23.e5f4f0a1.js"><link rel="prefetch" href="/assets/js/24.239130eb.js"><link rel="prefetch" href="/assets/js/25.db493ad9.js"><link rel="prefetch" href="/assets/js/26.26f3df98.js"><link rel="prefetch" href="/assets/js/27.d55bc04b.js"><link rel="prefetch" href="/assets/js/28.ac3a437a.js"><link rel="prefetch" href="/assets/js/29.9abce28f.js"><link rel="prefetch" href="/assets/js/30.ea843119.js"><link rel="prefetch" href="/assets/js/31.30a083bf.js"><link rel="prefetch" href="/assets/js/32.5f0b6b12.js"><link rel="prefetch" href="/assets/js/33.11456c83.js"><link rel="prefetch" href="/assets/js/34.e9ede601.js"><link rel="prefetch" href="/assets/js/35.17e5bb78.js"><link rel="prefetch" href="/assets/js/36.2f441457.js"><link rel="prefetch" href="/assets/js/37.e9fa7538.js"><link rel="prefetch" href="/assets/js/38.6bd1c237.js"><link rel="prefetch" href="/assets/js/39.8b0b956e.js"><link rel="prefetch" href="/assets/js/4.3ad057db.js"><link rel="prefetch" href="/assets/js/40.89ef1936.js"><link rel="prefetch" href="/assets/js/41.48d0560d.js"><link rel="prefetch" href="/assets/js/42.f804c44f.js"><link rel="prefetch" href="/assets/js/43.9dc6d003.js"><link rel="prefetch" href="/assets/js/45.60834ce1.js"><link rel="prefetch" href="/assets/js/46.c7eaf70d.js"><link rel="prefetch" href="/assets/js/47.a618490c.js"><link rel="prefetch" href="/assets/js/48.39a68335.js"><link rel="prefetch" href="/assets/js/49.1fa3bd55.js"><link rel="prefetch" href="/assets/js/5.4b2144a0.js"><link rel="prefetch" href="/assets/js/50.0ad5db28.js"><link rel="prefetch" href="/assets/js/51.843c519b.js"><link rel="prefetch" href="/assets/js/52.47d63846.js"><link rel="prefetch" href="/assets/js/53.5284b79d.js"><link rel="prefetch" href="/assets/js/54.b10b8ac1.js"><link rel="prefetch" href="/assets/js/55.3780915d.js"><link rel="prefetch" href="/assets/js/56.4a811b5c.js"><link rel="prefetch" href="/assets/js/57.228652ee.js"><link rel="prefetch" href="/assets/js/58.ef55d2ec.js"><link rel="prefetch" href="/assets/js/59.cbb81185.js"><link rel="prefetch" href="/assets/js/6.3dafc094.js"><link rel="prefetch" href="/assets/js/60.5257b363.js"><link rel="prefetch" href="/assets/js/61.03db625c.js"><link rel="prefetch" href="/assets/js/62.5982e690.js"><link rel="prefetch" href="/assets/js/63.0eccb44b.js"><link rel="prefetch" href="/assets/js/64.d92985de.js"><link rel="prefetch" href="/assets/js/65.4fd6aed1.js"><link rel="prefetch" href="/assets/js/66.acaf73d6.js"><link rel="prefetch" href="/assets/js/67.6db75c01.js"><link rel="prefetch" href="/assets/js/68.1dce863f.js"><link rel="prefetch" href="/assets/js/69.4222a30b.js"><link rel="prefetch" href="/assets/js/7.be8adaa3.js"><link rel="prefetch" href="/assets/js/8.3121f1c6.js"><link rel="prefetch" href="/assets/js/9.03dc3458.js">
    <link rel="stylesheet" href="/assets/css/0.styles.152c4062.css">
  </head>
  <body>
    <div id="app" data-server-rendered="true"><div class="theme-container" data-v-2d5f533b><div data-v-2d5f533b><div id="loader-wrapper" class="loading-wrapper" data-v-d48f4d20 data-v-2d5f533b data-v-2d5f533b><div class="loader-main" data-v-d48f4d20><div data-v-d48f4d20></div><div data-v-d48f4d20></div><div data-v-d48f4d20></div><div data-v-d48f4d20></div></div> <!----> <!----></div> <div class="password-shadow password-wrapper-out" style="display:none;" data-v-64685f0e data-v-2d5f533b data-v-2d5f533b><h3 class="title" style="display:none;" data-v-64685f0e data-v-64685f0e>伊水河的博客</h3> <!----> <label id="box" class="inputBox" style="display:none;" data-v-64685f0e data-v-64685f0e><input type="password" value="" data-v-64685f0e> <span data-v-64685f0e>Konck! Knock!</span> <button data-v-64685f0e>OK</button></label> <div class="footer" style="display:none;" data-v-64685f0e data-v-64685f0e><span data-v-64685f0e><i class="iconfont reco-theme" data-v-64685f0e></i> <a target="blank" href="https://vuepress-theme-reco.recoluan.com" data-v-64685f0e>vuePress-theme-reco</a></span> <span data-v-64685f0e><i class="iconfont reco-copyright" data-v-64685f0e></i> <a data-v-64685f0e><span data-v-64685f0e>伊水河</span>
            
          <span data-v-64685f0e>2020 - </span>
          2021
        </a></span></div></div> <div class="hide" data-v-2d5f533b><header class="navbar" data-v-2d5f533b><div class="sidebar-button"><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" role="img" viewBox="0 0 448 512" class="icon"><path fill="currentColor" d="M436 124H12c-6.627 0-12-5.373-12-12V80c0-6.627 5.373-12 12-12h424c6.627 0 12 5.373 12 12v32c0 6.627-5.373 12-12 12zm0 160H12c-6.627 0-12-5.373-12-12v-32c0-6.627 5.373-12 12-12h424c6.627 0 12 5.373 12 12v32c0 6.627-5.373 12-12 12zm0 160H12c-6.627 0-12-5.373-12-12v-32c0-6.627 5.373-12 12-12h424c6.627 0 12 5.373 12 12v32c0 6.627-5.373 12-12 12z"></path></svg></div> <a href="/" class="home-link router-link-active"><!----> <span class="site-name">伊水河的博客</span></a> <div class="links"><div class="color-picker"><a class="color-button"><i class="iconfont reco-color"></i></a> <div class="color-picker-menu" style="display:none;"><div class="mode-options"><h4 class="title">Choose mode</h4> <ul class="color-mode-options"><li class="dark">dark</li><li class="auto active">auto</li><li class="light">light</li></ul></div></div></div> <div class="search-box"><i class="iconfont reco-search"></i> <input aria-label="Search" autocomplete="off" spellcheck="false" value=""> <!----></div> <nav class="nav-links can-hide"><div class="nav-item"><a href="/categories/前端/" class="nav-link"><i class="iconfont reco-blog"></i>
  前端
</a></div><div class="nav-item"><a href="/categories/leetcode/" class="nav-link"><i class="iconfont reco-coding"></i>
  力扣
</a></div><div class="nav-item"><a href="/tag/" class="nav-link"><i class="iconfont reco-tag"></i>
  标签
</a></div><div class="nav-item"><a href="/note/" class="nav-link router-link-active"><i class="iconfont reco-suggestion"></i>
  笔记
</a></div><div class="nav-item"><a href="/todo/" class="nav-link"><i class="iconfont reco-document"></i>
  TODO
</a></div><div class="nav-item"><div class="dropdown-wrapper"><a class="dropdown-title"><span class="title"><i class="iconfont reco-account"></i>
      关于
    </span> <span class="arrow right"></span></a> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="https://juejin.cn/user/571401777456621" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont reco-juejin"></i>
  掘金
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li><li class="dropdown-item"><!----> <a href="https://yishuihe.gitee.io" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont reco-mayun"></i>
  码云
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li><li class="dropdown-item"><!----> <a href="https://github.com/lixiang89" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont reco-github"></i>
  github
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li><li class="dropdown-item"><!----> <a href="https://node.kg.qq.com/personal?uid=659d9d802d253e8231" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont "></i>
  全民K歌
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li></ul></div></div> <!----></nav></div></header> <div class="sidebar-mask" data-v-2d5f533b></div> <aside class="sidebar" data-v-2d5f533b><div class="personal-info-wrapper" data-v-ca798c94 data-v-2d5f533b><img src="/b.jpg" alt="author-avatar" class="personal-img" data-v-ca798c94> <h3 class="name" data-v-ca798c94>
    伊水河
  </h3> <div class="num" data-v-ca798c94><div data-v-ca798c94><h3 data-v-ca798c94>51</h3> <h6 data-v-ca798c94>文章</h6></div> <div data-v-ca798c94><h3 data-v-ca798c94>13</h3> <h6 data-v-ca798c94>标签</h6></div></div> <hr data-v-ca798c94></div> <nav class="nav-links"><div class="nav-item"><a href="/categories/前端/" class="nav-link"><i class="iconfont reco-blog"></i>
  前端
</a></div><div class="nav-item"><a href="/categories/leetcode/" class="nav-link"><i class="iconfont reco-coding"></i>
  力扣
</a></div><div class="nav-item"><a href="/tag/" class="nav-link"><i class="iconfont reco-tag"></i>
  标签
</a></div><div class="nav-item"><a href="/note/" class="nav-link router-link-active"><i class="iconfont reco-suggestion"></i>
  笔记
</a></div><div class="nav-item"><a href="/todo/" class="nav-link"><i class="iconfont reco-document"></i>
  TODO
</a></div><div class="nav-item"><div class="dropdown-wrapper"><a class="dropdown-title"><span class="title"><i class="iconfont reco-account"></i>
      关于
    </span> <span class="arrow right"></span></a> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="https://juejin.cn/user/571401777456621" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont reco-juejin"></i>
  掘金
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li><li class="dropdown-item"><!----> <a href="https://yishuihe.gitee.io" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont reco-mayun"></i>
  码云
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li><li class="dropdown-item"><!----> <a href="https://github.com/lixiang89" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont reco-github"></i>
  github
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li><li class="dropdown-item"><!----> <a href="https://node.kg.qq.com/personal?uid=659d9d802d253e8231" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont "></i>
  全民K歌
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li></ul></div></div> <!----></nav> <ul class="sidebar-links"><li><a href="/note/professionalJavascript4/" aria-current="page" class="sidebar-link">前言</a></li><li><a href="/note/professionalJavascript4/2.html" class="sidebar-link">第二章 HTML中的javascript</a></li><li><a href="/note/professionalJavascript4/3.html" class="sidebar-link">第三章 语言基础</a></li><li><a href="/note/professionalJavascript4/4.html" class="sidebar-link">第四章 变量、作用域、内存</a></li><li><a href="/note/professionalJavascript4/5.html" class="sidebar-link">第五章 基本引用类型</a></li><li><a href="/note/professionalJavascript4/6.html" class="sidebar-link">第六章 集合引用类型</a></li><li><a href="/note/professionalJavascript4/7.html" class="sidebar-link">第七章 迭代器和生成器</a></li><li><a href="/note/professionalJavascript4/8.html" class="sidebar-link">第八章 对象、类与面向对象编程</a></li><li><a href="/note/professionalJavascript4/9.html" class="sidebar-link">第九章 代理与反射</a></li><li><a href="/note/professionalJavascript4/10.html" aria-current="page" class="active sidebar-link">第十章 函数</a></li><li><a href="/note/professionalJavascript4/11.html" class="sidebar-link">第十一章 期约（Promise）与异步（async）函数</a></li><li><a href="/note/professionalJavascript4/12.html" class="sidebar-link">第十二章 BOM</a></li><li><a href="/note/professionalJavascript4/13.html" class="sidebar-link">第十三章 客户端检测</a></li><li><a href="/note/professionalJavascript4/14.html" class="sidebar-link">第十四/十五/十六章 DOM</a></li><li><a href="/note/professionalJavascript4/17.html" class="sidebar-link">第十七章 事件</a></li><li><a href="/note/professionalJavascript4/18.html" class="sidebar-link">第十七章 动画与canvas</a></li><li><a href="/note/professionalJavascript4/19.html" class="sidebar-link">第十九章 表单脚本</a></li><li><a href="/note/professionalJavascript4/24.html" class="sidebar-link">第二十四章 网络请求与远程资源</a></li><li><a href="/note/professionalJavascript4/25.html" class="sidebar-link">第二十五章 客户端存储</a></li><li><a href="/note/professionalJavascript4/26.html" class="sidebar-link">第二十六章 模块</a></li><li><a href="/note/professionalJavascript4/27.html" class="sidebar-link">第二十七章 工作者线程（worker）</a></li></ul> </aside> <div class="password-shadow password-wrapper-in" style="display:none;" data-v-64685f0e data-v-2d5f533b><h3 class="title" style="display:none;" data-v-64685f0e data-v-64685f0e>第十章 函数</h3> <!----> <label id="box" class="inputBox" style="display:none;" data-v-64685f0e data-v-64685f0e><input type="password" value="" data-v-64685f0e> <span data-v-64685f0e>Konck! Knock!</span> <button data-v-64685f0e>OK</button></label> <div class="footer" style="display:none;" data-v-64685f0e data-v-64685f0e><span data-v-64685f0e><i class="iconfont reco-theme" data-v-64685f0e></i> <a target="blank" href="https://vuepress-theme-reco.recoluan.com" data-v-64685f0e>vuePress-theme-reco</a></span> <span data-v-64685f0e><i class="iconfont reco-copyright" data-v-64685f0e></i> <a data-v-64685f0e><span data-v-64685f0e>伊水河</span>
            
          <span data-v-64685f0e>2020 - </span>
          2021
        </a></span></div></div> <div data-v-2d5f533b><main class="page"><div class="page-title" style="display:none;"><h1 class="title">第十章 函数</h1> <div data-v-3b7f5bdf><i class="iconfont reco-account" data-v-3b7f5bdf><span data-v-3b7f5bdf>伊水河</span></i> <i class="iconfont reco-date" data-v-3b7f5bdf><span data-v-3b7f5bdf>2020-12-18</span></i> <i class="iconfont reco-eye" data-v-3b7f5bdf><span id="/note/professionalJavascript4/10.html" data-flag-title="Your Article Title" class="leancloud-visitors" data-v-3b7f5bdf><a class="leancloud-visitors-count" style="font-size:.9rem;font-weight:normal;color:#999;"></a></span></i> <i class="iconfont reco-tag tags" data-v-3b7f5bdf><span class="tag-item" data-v-3b7f5bdf>javascript高级程序设计</span></i></div></div> <div class="theme-reco-content content__default" style="display:none;"><h2 id="箭头函数"><a href="#箭头函数" class="header-anchor">#</a> 箭头函数</h2> <p>箭头函数虽然语法简洁，但是不能使用<code>arguments</code>、<code>super</code>和<code>new.target</code>，也不能用作构造函数。此外，箭头函数也没有<code>prototype</code>属性。</p> <h2 id="函数名"><a href="#函数名" class="header-anchor">#</a> 函数名</h2> <p>ECMAScript 6的所有函数对象都会暴露一个只读的<code>name</code>属性，其中包含关于函数的信息。多数情况下，这个属性中保存的就是一个函数标识符，或者说是一个字符串化的变量名。即使函数没有名称，也会如实显示成空字符串。如果它是使用<code>Function</code>构造函数创建的，则会标识成<code>anonymous</code></p> <h2 id="默认参数"><a href="#默认参数" class="header-anchor">#</a> 默认参数</h2> <p>因为在求值默认参数时可以定义对象，也可以动态调用函数，所以函数参数肯定是在某个作用域中求值的。</p> <p>给多个参数定义默认值实际上跟使用<code>let</code>关键字顺序声明变量一样：</p> <div class="language-javascript extra-class"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">makeKing</span><span class="token punctuation">(</span>name<span class="token operator">=</span><span class="token string">&quot;Henry&quot;</span><span class="token punctuation">,</span>numerals<span class="token operator">=</span><span class="token string">&quot;VIII&quot;</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">King </span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>name<span class="token interpolation-punctuation punctuation">}</span></span><span class="token string"> </span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>numerals<span class="token interpolation-punctuation punctuation">}</span></span><span class="token template-punctuation string">`</span></span>
<span class="token punctuation">}</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token function">makeKing</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token comment">// King Henry VIII</span>
</code></pre></div><p>这里的默认参数会按照定义他们的顺序依次呗初始化。可以依照如下示例想象一下这个过程：</p> <div class="language-javascript extra-class"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">makeKing</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">let</span> name<span class="token operator">=</span><span class="token string">'Henry'</span>
    <span class="token keyword">let</span> numerals<span class="token operator">=</span><span class="token string">'VIII'</span>
    <span class="token keyword">return</span> <span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">King </span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>name<span class="token interpolation-punctuation punctuation">}</span></span><span class="token string"> </span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>numerals<span class="token interpolation-punctuation punctuation">}</span></span><span class="token template-punctuation string">`</span></span>
<span class="token punctuation">}</span>
</code></pre></div><p>因为参数是按顺序初始化的，所以后定义默认值的参数可以引用先定义的参数。看下面这个例子：</p> <div class="language-javascript extra-class"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">makeKing</span><span class="token punctuation">(</span>name<span class="token operator">=</span><span class="token string">&quot;Henry&quot;</span><span class="token punctuation">,</span>numerals<span class="token operator">=</span>name<span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">King </span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>name<span class="token interpolation-punctuation punctuation">}</span></span><span class="token string"> </span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>numerals<span class="token interpolation-punctuation punctuation">}</span></span><span class="token template-punctuation string">`</span></span>
<span class="token punctuation">}</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token function">makeKing</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token comment">// King Henry Henry</span>
</code></pre></div><p>如果前面的引用后面的则会报错。暂时性死区。</p> <h2 id="this"><a href="#this" class="header-anchor">#</a> this</h2> <p>在标准函数中，<code>this</code>引用的是把函数当成方法调用的上下文对象，这时候通常称其为<code>this</code>值。<code>this</code>到底引用哪个对象必须到函数被调用时才能确定。</p> <p>在箭头函数中，<code>this</code>引用的是定义箭头函数的上下文。</p> <p>如果函数没有使用箭头函数定义，则<code>this</code>对象会在运行时绑定到执行函数的上下文。如果作为某个对象的方法调用，则<code>this</code>等于这个对象。在某些情况下，<code>this</code>值可能并不是我们所期待的值。如：</p> <div class="language-javascript extra-class"><pre class="language-javascript"><code>window<span class="token punctuation">.</span>identity<span class="token operator">=</span><span class="token string">'window'</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> object<span class="token operator">=</span><span class="token punctuation">{</span>
    identity<span class="token operator">:</span><span class="token string">'object'</span><span class="token punctuation">,</span>
    <span class="token function">getIdentity</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>identity<span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

object<span class="token punctuation">.</span><span class="token function">getIdentity</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 'object'</span>
<span class="token punctuation">(</span>object<span class="token punctuation">.</span>getIdentity<span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 'object'</span>
<span class="token punctuation">(</span>object<span class="token punctuation">.</span>getIdentity<span class="token operator">=</span>object<span class="token punctuation">.</span>getIdentity<span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// window</span>
</code></pre></div><p>第一行调用<code>object.getIdentity()</code>是正常调用，返回<code>object</code>，因此<code>this.identity</code>就是<code>object.identity</code>。
第二行在调用时吧<code>object.getIdentity</code>放在了括号里。虽然看起来是对一个函数的引用，但<code>this</code>值并没有改变。只是因为按照规范，<code>object.getIdentity</code>和<code>(object.getIdentity)</code>是相等的。
第三行执行了一次赋值，然后再调用赋值后的结果。因为赋值表达式的值是函数本身，<code>this</code>值不在与任何对象绑定，所以返回的是<code>window</code>。</p> <ul><li><code>自备</code>：个人理解：第三次调用，因为赋值操作返回的是一个函数，而执行时的上下文又是全局上下文，浏览器环境即是在<code>window</code>上调用<code>getIdentity()</code>，所以this值为<code>window</code>。</li></ul> <p>一般情况下，不大可能像第二行和第三行这样的写法。但是通过这个例子，我们可以知道，即使语法不同，也可能影响this的值。</p> <h2 id="new-target"><a href="#new-target" class="header-anchor">#</a> new.target</h2> <p>ECMAScript中的函数始终可以作为构造函数实例化一个新对象，也可以作为普通函数被调用。ECMAScript 6新增了检测函数是否使用<code>new</code>关键字调用的<code>new.target</code>属性。如果函数是正常调用的，则<code>new.target</code>的值是<code>undefined</code>；如果是使用<code>new</code>关键字调用的，则<code>new.target</code>将引用被调用的构造函数。</p> <div class="language-javascript extra-class"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">King</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">if</span><span class="token punctuation">(</span><span class="token operator">!</span><span class="token keyword">new</span><span class="token punctuation">.</span>target<span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token keyword">throw</span> <span class="token string">'King must be instantiated using &quot;new&quot;'</span>
    <span class="token punctuation">}</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'King instantiated using &quot;new&quot;'</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>

<span class="token keyword">new</span> <span class="token class-name">King</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">//King instantiated using &quot;new&quot;</span>
<span class="token function">King</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">//Error:King must be instantiated using &quot;new&quot;</span>
</code></pre></div><h2 id="属性与方法"><a href="#属性与方法" class="header-anchor">#</a> 属性与方法</h2> <ul><li><p>length：命名参数的个数</p></li> <li><p>prototype：保存了引用类型所有实例方法的地方，由所有实例共享。在ECMAScript 5中，<code>prototype</code>属性时不可枚举的，因此使用<code>for-in</code>循环不会返回这个属性。</p></li> <li><p>call()：第一个参数是<code>this</code>值，其余的是要传给被调用函数的参数，逐个传递。</p></li> <li><p>apply()：两个参数，第一个是<code>this</code>值，第二个是数组。</p></li> <li><p>bind()：返回一个新的函数，其<code>this</code>值会被绑定给第一个参数，其余参数则逐个传递给被调用函数。</p></li> <li><p>toString()：返回函数的代码。</p></li> <li><p>valueOf()：返回函数本身。</p></li></ul></div> <footer class="page-edit" style="display:none;"><!----> <!----></footer> <!----> <!----> <!----></main> <!----></div></div></div></div><div class="global-ui"><div class="back-to-ceiling" style="right:1rem;bottom:6rem;width:2.5rem;height:2.5rem;border-radius:.25rem;line-height:2.5rem;display:none;" data-v-c6073ba8 data-v-c6073ba8><svg t="1574745035067" viewBox="0 0 1024 1024" version="1.1" xmlns="http://www.w3.org/2000/svg" p-id="5404" class="icon" data-v-c6073ba8><path d="M526.60727968 10.90185116a27.675 27.675 0 0 0-29.21455937 0c-131.36607665 82.28402758-218.69155461 228.01873535-218.69155402 394.07834331a462.20625001 462.20625001 0 0 0 5.36959153 69.94390903c1.00431239 6.55289093-0.34802892 13.13561351-3.76865779 18.80351572-32.63518765 54.11355614-51.75690182 118.55860487-51.7569018 187.94566865a371.06718723 371.06718723 0 0 0 11.50484808 91.98906777c6.53300375 25.50556257 41.68394495 28.14064038 52.69160883 4.22606766 17.37162448-37.73630017 42.14135425-72.50938081 72.80769204-103.21549295 2.18761121 3.04276886 4.15646224 6.24463696 6.40373557 9.22774369a1871.4375 1871.4375 0 0 0 140.04691725 5.34970492 1866.36093723 1866.36093723 0 0 0 140.04691723-5.34970492c2.24727335-2.98310674 4.21612437-6.18497483 6.3937923-9.2178004 30.66633723 30.70611158 55.4360664 65.4791928 72.80769147 103.21549355 11.00766384 23.91457269 46.15860503 21.27949489 52.69160879-4.22606768a371.15156223 371.15156223 0 0 0 11.514792-91.99901164c0-69.36717486-19.13165746-133.82216804-51.75690182-187.92578088-3.42062944-5.66790279-4.76302748-12.26056868-3.76865837-18.80351632a462.20625001 462.20625001 0 0 0 5.36959269-69.943909c-0.00994388-166.08943902-87.32547796-311.81420293-218.6915546-394.09823051zM605.93803103 357.87693858a93.93749974 93.93749974 0 1 1-187.89594924 6.1e-7 93.93749974 93.93749974 0 0 1 187.89594924-6.1e-7z" p-id="5405" data-v-c6073ba8></path><path d="M429.50777625 765.63860547C429.50777625 803.39355007 466.44236686 1000.39046097 512.00932183 1000.39046097c45.56695499 0 82.4922232-197.00623328 82.5015456-234.7518555 0-37.75494459-36.9345906-68.35043303-82.4922232-68.34111062-45.57627738-0.00932239-82.52019037 30.59548842-82.51086798 68.34111062z" p-id="5406" data-v-c6073ba8></path></svg></div><APlayer audio="" fixed="true" theme="#b7daff" loop="loop" order="random" preload="auto" volume="0.7" mutex="true" lrc-type="3" list-folded="true" list-max-height="250" storage-name="vuepress-plugin-meting" id="aplayer-fixed"></APlayer></div></div>
    <script src="/assets/js/app.2d22f3f9.js" defer></script><script src="/assets/js/3.6721f6a0.js" defer></script><script src="/assets/js/1.546ea5a7.js" defer></script><script src="/assets/js/44.3033d12c.js" defer></script>
  </body>
</html>
